Satellite communication system

ABSTRACT

An earth station transmitter device is arranged for generating a signal to be transmitted to a plurality of earth station receiver devices of a satellite communication system. The earth station transmitter device comprises a plurality of shapers, each arranged for shaping and encapsulating data traffic to a different subset of earth station receiver devices; obtaining for each subset a virtual carrier outputting at a virtual carrier symbol rate a plurality of virtual carrier baseband frames. A modulator includes a time slice selector arranged for receiving and storing the virtual carriers outputted by the plurality of shapers, for selecting a stored virtual carrier baseband frame from a list of allowable virtual carriers as next frame to be multiplexed on a single stream, and for assigning to the selected virtual carrier baseband frame, from a list of allowable time slice numbers.

FIELD OF THE INVENTION

The present invention is generally related to the field of satellitecommunication systems.

BACKGROUND OF THE INVENTION

Satellite communication services are important in various fields sincemany decades. Consider for example Internet over satellite forconsumers, but also for enterprises (e.g. oil rigs) and government anddefense applications.

One-way and two-way communication services are considered. In two-waysatellite communication services there is a link from a hub to aterminal, called the forward (FWD) link, and a link from the terminal tothe hub, called the return (RTN) link. In a one-way service, only theFWD link is used. A typical realization of data communication over a(two-way) satellite network is based on a star network as illustrated inFIG. 1. A hub or gateway (1) communicates with a terminal (3) via atleast one satellite (2). In such a system multiple terminals may becovered through a single hub. A satellite communication service maycontain several hubs. A hub may contain several transmitters and/orreceivers, e.g. if the bandwidth needed in the forward link is largerthan the bandwidth that can be transmitted from a single transmitter.The receive terminals to which the traffic can potentially be combinedin a single frame, are grouped in a satellite network, referred to as asatnet. These terminals decode a same carrier at the same time in a samecontour. A satnet processor (e.g. a central processing unit (CPU)processor on a blade server) is responsible for handling forward andreturn (also referred to as inbound and outbound) traffic associated toa satnet. A blade server is a stripped-down server computer with amodular design optimized to minimize the use of physical space andenergy.

A satellite communication system is considered wherein in the forwardlink a satnet processor (e.g. a processor on a blade server) multiplexesdata (also referred to as traffic) to a group of terminals in a framewhich is then sent to the modulator (e.g. over an Ethernet cable or overa coaxial cable). Such a frame is for example a baseband frame but itcan also be another type of frame. The two essential components of thesatnet processor are referred to as a shaper and an encapsulator. Theaverage speed or rate at which such frames are sent to the modulator,depends on the average rate at which data for this satnet is transmittedover the air (typically equal to a symbol rate of a transmitted carrieror a fraction thereof in the case of time slicing, see DVB-S2 Annex M).

In the transmitter part of prior art satellite communication systems ashaper-encapsulator shapes a physical carrier symbol rate, i.e., itsends sufficient baseband frames (in the form of a sequence of bits) tothe modulator such that the modulator can encode and modulate the bitsonto an RF carrier at a particular symbol rate to fill the physicalcarrier without buffer overflow in the modulator. Thus, one physicalcarrier is used for data traffic of exactly one satnet. The traffic frommultiple users in exactly one satnet is multiplexed in the same carrier,whereby the multiplexing takes into account jitter requirements, trafficpriority classes, the adoption of a beam hopping or non-beam hoppingsatellite, based on link budgets and on the symbol rate and the linkbudget per user. This complex process takes place in the satnetprocessor. The process is referred to as shaping-encapsulation, asdiscussed e.g. in WO2006/099695. The modulator then encodes andmodulates the baseband frames into physical layer frames consisting ofcomplex-valued symbols (at a rate equal to the physical carrier symbolrate) and finally an RF signal (typically with a 3-dB bandwidth equal tothe physical carrier symbol rate when using Nyquist signaling).

Time slicing was standardized In ETSI EN 302 307: “Digital VideoBroadcasting (DVB); Second generation framing structure, channel codingand modulation systems for Broadcasting, Interactive Services, NewsGathering and other broadband satellite applications”, annex M) and alsoin ETSI EN 302 307-2 V1.1.1 (2014-10): “Digital Video Broadcasting(DVB); Second generation framing structure, channel coding andmodulation systems for Broadcasting, Interactive Services, NewsGathering and other broadband satellite applications; Part 2: DVB-S2Extensions (DVB-S2X)”, here-after referred to as DVB-S2X. In the sharingmechanism implemented by time slicing, receivers only decode frameswhich have a frame tag or slice number associated to the group ofreceivers they belong to.

The need for time slicing has come through the introduction of verywideband transmissions over high throughput satellites. Very highthroughput satellites (HTS), where single transponders have largebandwidths (in the order of 500 MHz), have significantly reduced thecost per Mbit for satellite communications and are therefore omnipresentin applications like broadband, mobile and enterprise connectivity (seeO. Vidal et al., “Next generation high throughput satellite system,”IEEE AESS Eur. Conf. on Sat. Telecom. (ESTEL), 2012). The data demand ina satellite network is typically greatest in the forward link fromgateway over satellite to terminal, as users have higher downloaddemands than upload demands. Thus, the number of subscribers is limiteddue to the available bandwidth in the forward link. Hence, theachievable bandwidth of the forward link carrier is a differentiator.This bandwidth has increased dramatically with the introduction of HTS.The consequence, however, is that all receivers in the satellite networkmust be able to process this huge bandwidth, which is illustrated inFIG. 1. Typically, the throughput of the coded bits is limited due todecoder throughput limitations at the receiver. Also, the clock speed atwhich buffers are read is limited at receivers. Time slicing, asexplained in ETSI EN 302 307, has been proposed in EP2073400 B1. Whenperforming time slicing, receiver terminals are subdivided in groupswhich each only decode a subset of the frames, said subset of framesidentified through an associated frame tag or slice number. ETSI EN 302307 explains which encoder to use to encode the time slice number in thephysical layer header (the first 180 complex-valued symbols of aphysical layer frame, in the case of ETSI EN 302 307). This allows areceiver to only decode the physical layer header to decide whether itis required to further decode the entire frame or not.

However, it remains unclear how to organize a gateway in order to makesure there is sufficient time between two frames of the same slice, sothat receivers with decoders having lower throughput specifications haveno packet loss due to throughput violations. More specifically, ETSI EN302 307 does not standardize or explain shaper-encapsulators. It assumesthe input stream or streams at the modulator input to be such that thenumber of symbols per second to be transmitted are not higher than thesymbol rate of the physical carrier. Further, it does not explain how tomerge or multiplex baseband frames with different slice numbers into asingle stream of baseband frames. To avoid any misunderstanding, themerger/slicer explained in ETSI EN 302 307 does not mention anything ona minimum guard time between baseband frames with the same time slicenumber (where time slice number is referring to the time slicing inAnnex M).

Prior art solutions concerning shaper-encapsulators (e.g. as in theabove-mentioned WO2006/099695) have a 1-to-1 relation betweenshaper-encapsulators and physical carriers. That is, ashaper-encapsulator bursts frames at a rate that fits just in thephysical carrier symbol rate. It is not clear how such a prior artshaper-encapsulator has to meet a requirement that frames transmittedwith the same slice number be sufficiently spaced in time, g,ak: iventhat the shaper-encapsulator acts in a processor which is not in thesame clock domain as the modulator on the FPGA. Shaper-encapsulatorsfurthermore shape symbol rates while the constraints are expressed incoded throughput rates.

Further, the impact of transmitting consecutive frames with differenttime slice numbers but also different modcods, e.g. a 32-APSK and a QPSKmodcod is not clear either.

The beam hopping satellite communication system presented inWO2018/92132 exploits a transmit-receive framing mechanism to simplifyscheduling, streamline satellite and beam switchover. Most of thecomplexity involved in routing and handover is shifted from thesatellite to the gateway and the terminals. The communication channel istransmitted in burst communication mode such that transmission signalincludes transmission data time slots at which one or more of saidcommunication frames are encoded in the signal and one or more recesstime slots between them. The communication receiver is adapted forprocessing signals of the burst mode communication channel and processesat least a portion of a signal received in the communication channelafter a recess time period during which communication frames were nottransmitted to determine a carrier frequency of the communicationchannel.

Hence, there is a need to extend nowadays systems in a modular, simpleand cost efficient way so that they are able to address time slicing,without introducing new inefficiencies.

SUMMARY OF THE INVENTION

It is an object of embodiments of the present invention to provide for acost-effective transmitter reusing off-the-shelf solutions for asatellite communication system wherein time slicing can be appliedwithout causing inefficiencies.

The above objective is accomplished by the solution according to thepresent invention.

In a first aspect the invention relates to an earth station transmitterdevice arranged for generating a signal to be transmitted to a pluralityof earth station receiver devices of a satellite communication system.The earth station transmitter device comprises

-   a plurality of shaping means, each arranged for shaping and    encapsulating data traffic to a different subset of earth station    receiver devices, so obtaining for each subset a virtual carrier    outputting at an average equivalent symbol rate denoted a virtual    carrier symbol rate a plurality of virtual carrier baseband frames,-   a modulator comprising a time slice selector arranged for receiving    and storing said virtual carriers outputted by said plurality of    shaping means, for selecting a stored virtual carrier baseband frame    from a list of allowable virtual carriers as next frame to be    multiplexed on a single stream, and for assigning to said selected    virtual carrier baseband frame, from a list of allowable time slice    numbers, a time slice number associated to the virtual carrier to    which said selected virtual carrier baseband frame belongs, said    modulator further comprising encoding and modulation means to    convert said single stream into symbols of a continuous physical    carrier to be transmitted at a physical carrier symbol rate greater    than or equal to the sum of said virtual carrier symbol rates,-   a central unit arranged for conveying to each shaping means of said    plurality its virtual carrier symbol rate and for conveying to said    modulator a list of possible time slice numbers for each of said    virtual carriers.

The proposed solution indeed allows for performing time slicing. A timeslice selector in the modulator receives and stores virtual carrierbaseband frames from at least two virtual data carriers, which each havetheir own virtual carrier symbol rate. A central unit is in control andinforms the various shapers of the virtual carrier symbol rate that canbe applied. The shaping means, also referred to as shapers, performshaping and encapsulation. The time slice selector is further capable ofselecting one of the stored frames as next frame to be put on the singlestream and of assigning a time slice number to the selected virtualcarrier baseband frame. The encoding and modulation means then convertthe single stream into symbols of a continuous physical carrier to betransmitted. Continuous is hereby to be construed as without any guardtime: all frames are transmitted back to back. Note that in absence ofuser data, frames without useful data are inserted in the single streamin order to keep transmitting frames back to back. One of the mostinteresting features of applying time slicing is that by doing soreceivers get more time to decode frames. The proposed solution ismodular and cost-effective as off-the-shelf shapers are reused to shapeand encapsulate to a given, here virtual carrier, symbol rate. Alsooff-the-shelf encoding and modulation means are reused.

In preferred embodiments the selected virtual carrier baseband frame isthe virtual carrier baseband frame from said list of allowable virtualcarriers that was stored longest ago.

In certain embodiments the central unit is also arranged for conveyingto the time slice selector indications of throughput limits per timeslice number.

Advantageously, the encoding and modulation means is arranged to send arequest to the time slice selector to get the next selected virtualcarrier baseband frame. The time slice selector then provides,corresponding to the physical carrier symbol rate, sufficient basebandframes to the encoding and modulation means so that the encoding andmodulation means only transmit symbols corresponding to the basebandframes received from the time slice selector. In other words, theencoding and modulation means does not insert any extra frame.

In advantageous embodiments the earth station transmitter devicecomprises an Ethernet switch for receiving the virtual carriers and formultiplexing the virtual carrier baseband frames, said multiplexedvirtual carrier baseband frames being sent over a single Ethernet cableto the modulator.

In one embodiment the plurality of shaping encapsulation means isarranged to provide output in data packets, each data packet comprisingone or an integer multiple of virtual carrier baseband frames.

In embodiments of the invention the virtual carrier symbol rate of atleast one of the virtual carriers is upper bounded depending on the mostefficient modulation and coding used in that virtual carrier.

In another aspect the invention relates to a satellite communicationsystem comprising an earth station transmitter device as previouslydescribed and a plurality of earth station receiver devices, wherein atleast one earth station receiver device of that plurality is arranged todemodulate and decode the symbols of the physical carrier only at a ratelower than the physical carrier symbol rate, said plurality of earthstation receiver devices being divided in at least two subsets, eachsubset corresponding to a different one of the virtual carriers.

In a preferred embodiment the time slice selector is arranged forselecting a non-limited frame to be put in the single stream in case novirtual carrier baseband frame is found with allowable time slicenumber. Preferably the time slice selector is arranged to indicate tothe encoding and modulation means the type and number of non-limitedframes to be inserted after a particular virtual carrier baseband frame,in case no new virtual carrier baseband frame is found with allowabletime slice number. In an embodiment the non-limited frame is the oldeststored baseband frame belonging to a virtual carrier to be received byat least one earth station receiver device of the plurality arranged fordemodulation and decoding at the physical carrier symbol rate.

In embodiments the central unit is arranged to keep a separate list oftime slice numbers for the at least one earth station receiver devicearranged for demodulation and decoding at the physical carrier symbolrate. In absence of baseband frames for the at least one earth stationreceiver device arranged for demodulation and decoding at the physicalcarrier symbol rate, the non-limited frame may be a frame that is forcedinto the single stream by the time slice selector and that is rejectedby all earth station receiver devices.

In other embodiments the non-limited frame is a frame that is forcedinto the single stream by the time slice selector and is rejected by allearth station receiver devices, said frame comprising a headeridentifying the frame as a dummy frame. In some of the embodiments theframe forced into the single stream is a dummy frame if the physicalcarrier symbol rate is below a given threshold and a 16APSK or 32APSKnormal frame with dummy data above that given threshold.

In embodiments the frame forced into the single stream is a 16APSK or32APSK normal frame with dummy data. The 16APSK or 32APSK normal framemay be a prestored, already coded frame with dummy data.

In embodiments of the invention at least one of the earth stationreceiver devices is arranged for demodulating and decoding all frames ofthe single stream.

In preferred embodiments the list of allowable time slice numberscomprises time slice numbers for which a counter exceeds a firstthreshold linked to a given throughput limitation per time slice on anearth station receiver device of the plurality, said counter arranged tocount a number of symbols transmitted as of and including the lastoutputted baseband frame assigned to the time slice number.

In certain embodiments the list of allowable virtual carriers comprisesvirtual carriers, most preferably all, having at least one time slicenumber on the list of allowable time slice numbers. The list of virtualcarriers comprises the virtual carriers for which a further counterexceeds a second threshold related to a jitter specification on an earthstation receiver device that can lead to frame reordering, said secondcounter arranged to count a number of symbols transmitted as of andincluding the last outputted baseband frame assigned to the virtualcarrier.

In embodiments of the invention the central unit is arranged to indicateto a first shaping-encapsulation means an increased virtual carriersymbol rate, based on congestion of a corresponding satnet and toindicate a decreased virtual carrier symbol rate to at least one othershaping-encapsulation means, such that the physical carrier symbol rateis greater than or equal to the sum of all virtual carrier symbol rates.

Advantageously the central unit is arranged to first indicate adecreased virtual carrier symbol rate to the at least one othershaping-encapsulation means, before indicating an increased virtualcarrier symbol rate to the first shaping-encapsulation means.

For purposes of summarizing the invention and the advantages achievedover the prior art, certain objects and advantages of the invention havebeen described herein above. Of course, it is to be understood that notnecessarily all such objects or advantages may be achieved in accordancewith any particular embodiment of the invention. Thus, for example,those skilled in the art will recognize that the invention may beembodied or carried out in a manner that achieves or optimizes oneadvantage or group of advantages as taught herein without necessarilyachieving other objects or advantages as may be taught or suggestedherein.

The above and other aspects of the invention will be apparent from andelucidated with reference to the embodiment(s) described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described further, by way of example, withreference to the accompanying drawings, wherein like reference numeralsrefer to like elements in the various figures.

FIG. 1 illustrates a satellite communications system where a hub orgateway (1) communicates with multiple terminals (3) via a satellite(2). The link from gateway to satellite in the forward (FWD) link istypically the bottleneck.

FIG. 2 illustrates the concept of time slicing.

FIG. 3 illustrates the use of one virtual carrier per satnet and themultiplexing of multiple virtual carriers (and satnets) in one physicalcarrier.

FIG. 4 illustrates the association of disjunct sets of time slicenumbers per virtual carrier.

FIG. 5 illustrates a block scheme of an embodiment of a modulator of theearth station transmitter device according to the invention.

FIG. 6 illustrates a flowchart of a preferred embodiment of the outputside process of the time slice selector, selecting a virtual carrierbaseband frame and corresponding time slice number as adopted in thepresent invention.

FIG. 7 illustrates a flowchart of another embodiment of the output sideprocess of the time slice selector, selecting a virtual carrier basebandframe and corresponding time slice number as adopted in the presentinvention.

FIG. 8 illustrates the force-empty efficiency vs the physical carriersymbol rate.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention will be described with respect to particularembodiments and with reference to certain drawings but the invention isnot limited thereto but only by the claims.

Furthermore, the terms first, second and the like in the description andin the claims, are used for distinguishing between similar elements andnot necessarily for describing a sequence, either temporally, spatially,in ranking or in any other manner. It is to be understood that the termsso used are interchangeable under appropriate circumstances and that theembodiments of the invention described herein are capable of operationin other sequences than described or illustrated herein.

It is to be noticed that the term “comprising”, used in the claims,should not be interpreted as being restricted to the means listedthereafter; it does not exclude other elements or steps. It is thus tobe interpreted as specifying the presence of the stated features,integers, steps or components as referred to, but does not preclude thepresence or addition of one or more other features, integers, steps orcomponents, or groups thereof. Thus, the scope of the expression “adevice comprising means A and B” should not be limited to devicesconsisting only of components A and B. It means that with respect to thepresent invention, the only relevant components of the device are A andB.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment of the present invention. Thus, appearances of the phrases“in one embodiment” or “in an embodiment” in various places throughoutthis specification are not necessarily all referring to the sameembodiment, but may. Furthermore, the particular features, structures orcharacteristics may be combined in any suitable manner, as would beapparent to one of ordinary skill in the art from this disclosure, inone or more embodiments.

Similarly it should be appreciated that in the description of exemplaryembodiments of the invention, various features of the invention aresometimes grouped together in a single embodiment, figure, ordescription thereof for the purpose of streamlining the disclosure andaiding in the understanding of one or more of the various inventiveaspects. This method of disclosure, however, is not to be interpreted asreflecting an intention that the claimed invention requires morefeatures than are expressly recited in each claim. Rather, as thefollowing claims reflect, inventive aspects lie in less than allfeatures of a single foregoing disclosed embodiment. Thus, the claimsfollowing the detailed description are hereby expressly incorporatedinto this detailed description, with each claim standing on its own as aseparate embodiment of this invention.

Furthermore, while some embodiments described herein include some butnot other features included in other embodiments, combinations offeatures of different embodiments are meant to be within the scope ofthe invention, and form different embodiments, as would be understood bythose in the art. For example, in the following claims, any of theclaimed embodiments can be used in any combination.

It should be noted that the use of particular terminology whendescribing certain features or aspects of the invention should not betaken to imply that the terminology is being re-defined herein to berestricted to include any specific characteristics of the features oraspects of the invention with which that terminology is associated.

In the description provided herein, numerous specific details are setforth. However, it is understood that embodiments of the invention maybe practiced without these specific details. In other instances,well-known methods, structures and techniques have not been shown indetail in order not to obscure an understanding of this description.

The invention presents an earth station transmitter device and satellitecommunication system comprising such an earth station transmitter devicethat forms a modular, simple and cost efficient extension of prior artsolutions so that they become able of applying time slicing.

More in particular, the invention proposes in a first aspect an earthstation transmitter device which is a scalable and cost efficientextension of transmitters as encountered in state of the art satellitecommunication systems, allows for wideband (500 Mbaud) transmission andstatistical multiplexing and is suitable for cooperating with receiverdevices having lower throughput specifications. If receivers are usedthat only allow lower throughput, such receivers need sufficient time todecode their carrier of interest. Hence, they cannot decode eachsubsequent frame of a physical carrier transmitted at 500 Mbaud.Therefore, receivers are divided into subsets referred to as a satellitenetwork (satnet), and multiple satnets are served by the same physicalcarrier. Frames from the same satellite network are assigned a frametag, denoted a time slice number, as already described previously. Thistime slice number is encoded in a header of the physical layer frame(also referred to as PL frame, see ETSI EN 302 307) and can be decodedseparately. When a receiver does not belong to the satellite networkcorresponding to a decoded time slice number, that receiver discards theframes sent with that time slice number. As such, receivers get moretime to decode the frames of the satellite network they belong to. FIG.2 provides an illustration. It shows that the processing time needed fordemodulation is much shorter in the case of wideband transmission (ofe.g. 500 Mbaud) than for narrowband transmission. Assigning a time slicenumber creates more time to decode a frame, just like in narrowbandtransmission. This considerably contributes to keeping the receiverdesign cost efficient. Summarizing, in time slicing the transmittedstream is cut in time slices. Each time slice carries, at eachoccurrence, exactly one physical layer frame (which is a frame of IQsymbols after encoding and modulating a baseband frame with onemodulation and coding or modcod, see ETSI EN 302 307) and is categorizedthrough a time slice number which is encoded in the physical layerheader. Receivers select time slices of the aggregate physical carrierby decoding the physical layer header to detect the time slice number.Receivers subsequently only decode physical layer frames with one ormore particular time slice numbers related to their satnet. Althoughtime slicing is standardized in ETSI EN 302 307, the same concept ofincluding a time slice number in the header of frame to inform receiverson a category the frame belongs to, applies to any block coded (whereframes are used, per definition) transmission scheme. In other words,the approach presented in this invention goes beyond merely the ETSI EN302 307 standard.

The transmitter device proposed in the present invention is arranged formultiplexing multiple satnets in one physical carrier of, for example,500 Mbaud. Hence, contrary to prior art approaches, several satnetsshare here the same physical carrier. More specifically, each satnetgets a portion of the physical carrier. Each portion then corresponds toa virtual carrier symbol rate. For example, in case three equally largevirtual carriers are sharing the same physical carrier, the virtualcarrier symbol rate is upper bounded by one third of the physicalcarrier symbol rate.

The plurality of remote receiver terminals of the satellitecommunication system to which the transmitted signal conveys informationdata, may contain various types of terminals. For example, there may beterminals (‘professional terminals’) which are able to demodulate anddecode consecutive frames at the physical carrier symbol rate, whileother terminals can only handle a throughput lower than that physicalcarrier symbol rate and therefore require sufficient time between eachframe with the same time slice number in order to be able to demodulateand decode them without packet loss. In general, every type of terminalcan come with its own limitation in throughput.

The following design is proposed in order to achieve a modular systemimplementation with reuse of existing blocks (which leads to thefastest, cost effective and most scalable implementation) and makingabstraction of the modified physical layer, which now includes timeslicing. Each satnet is treated independently and each satnet processorshapes the traffic of its satnet users to a certain symbol rate. Thissymbol rate, however, is not the physical carrier symbol rate but ratherthe symbol rate of a virtual carrier, denoted a virtual carrier symbolrate. A virtual carrier refers to the collection of time slices assignedto the corresponding satnet. FIG. 3 offers an illustration. Each virtualcarrier is matched to a satellite network (satnet) and corresponds toone or more time slice numbers (but not all). A receiver knows whichphysical layer frame to demodulate and decode to retrieve the basebandframe, by first decoding the time slice number encoded in the header ofeach physical layer frame. If the time slice number decoded is comprisedin the set of the time slice numbers associated to the satnet thereceiver belongs to, it demodulates and decodes the physical layerframe, otherwise it discards the physical layer frame. Independentsatnet processing before the modulation is performed per virtualcarrier. E.g., two time slice numbers can be assigned to a satnet, suchthat the virtual carrier is a collection of the frames corresponding tothose two time slice numbers. A time slice number can only be assignedto a single satnet, thus to a single virtual carrier. Hence, given aparticular time slice number it is immediately known which satnet isconsidered. The virtual carriers each relate to a disjunct set of N timeslice numbers. This is illustrated in FIG. 4. The number N of time slicenumbers per virtual carrier is not necessarily the same for each virtualcarrier. Suppose that, in time, those two slices occupy 20% of thetransmitted baseband symbols. The virtual carrier symbol rate for thatvirtual carrier is then 20% of the aggregate physical carrier symbolrate. This concept of virtual carriers of course only works in thepresence of specific bounds on the virtual carrier symbol rates in orderto avoid packet loss, as discussed more in detail below. Those specificbounds, as well as the association of time slice numbers with groups ofreceivers, are applied by a controller (i.e. a central unit) which issteering the shaper-encapsulators. Whether the symbol rate shaped to isa virtual carrier symbol rate or a physical carrier symbol rate makes nodifference for the shaper-encapsulator. Whether time slicing is used ornot makes no difference. In other words, the existing off-the-shelfshaper-encapsulators can be reused.

Next, a mechanism is needed to multiplex the baseband frames from themultiple shaper-encapsulators. Indeed, for e.g. three virtual carriersin a physical carrier, three shaper-encapsulators burst baseband framesto a single modulator.

Hence, for all layers above the physical layer, it is as if thetransmission occurs over multiple carriers. In other words, whether datais transmitted over multiple carriers or a single carrier is no morethan a physical layer implementation detail. In upper layers the furtherprocessing is modular and done on satnet per satnet basis.

The satnet processors in the transmitter device, which comprise theshapers, i.e. the shaping and encapsulating means, are configurable. Forexample, a service provider can statically configure its network, e.g.by configuring the virtual carrier symbol rates for each satnet. In anembodiment the central unit can dynamically change the virtual carriersymbol rates within a physical carrier taking into account congestion,jitter and other important constraints in the network. To avoid bufferoverflow, it is important to first lower the symbol rate of the virtualcarriers that get smaller, after which the symbol rate of the virtualcarriers that get bigger can be increased.

The shapers, which perform shaping and encapsulation and each correspondto a virtual carrier, send UDP or other type of packets to an Ethernetswitch. Each packet contains one or more full baseband frames. Theswitch multiplexes the incoming packets on one output bit stream to themodulator. As a baseband frame is not spread over multiple packets, theoutput bit stream to the modulator contains a sequence of full basebandframes such that no reassembling on the modulator is needed. This savesresources and implementation time. In the modulator a time sliceselector stores the baseband frames in storage means, e.g. in a RAM. Theinvention of course does not exclude other ways to bring baseband framesto the modulator, e.g. by allowing baseband frames to be cut in piecesrequiring for a reassembling of frames on the modulator first or byreceiving frames from the means for shaping-encapsulating over anotherinterface than an Ethernet cable, such as a coaxial cables, subsequentlystoring the frames of all virtual carriers in the storage means.

The most important requirement for timesliced transmission to at least agroup of receivers that cannot handle a throughput corresponding to thephysical carrier rate but only a lower throughput, is to respect therespective specs in throughput from those receivers. For example, thereceivers may not be able to decode more than 800 Mcbps (Mega coded bitsper second). This holds for the set of time slices numbers decoded bythat particular type of receivers. E.g., a receiver belonging tosatellite network 3 decodes virtual carrier 3, which corresponds toslice numbers 5, 8 and 12. The instantaneous throughput of those slicescannot be larger than 800 Mcbps. Next, other receiver limitations mayapply, e.g. a per slice throughput limitation due to limited buffersizes or clock speeds in the time slice selector for example. As aconsequence, the modulator must have full control on the time durationbetween the transmission of frames of the same slice, more specificallyon the time interval between two frames with the same slice number. Inaddition, it is in general not desirable that two frames with differentslice numbers but in the same virtual carrier, e.g. slices 5, 8 and 12in the above example, be reordered in the receiver (e.g. reordering dueto different processing time in the receiver for the two slices).Therefore, it may be useful to also impose a sufficient time durationbetween two frames of the same virtual carrier, even if they do notbelong to the same slice number.

On the other hand, having too much time between two frames of the sameslice number or the same virtual carrier leads to an unnecessarily lowallowed throughput for that slice or virtual carrier. In order tomaximize satnet sizes this obviously is to be avoided as well. Thus,again, a modulator must have full control on the time interval betweentwo frames of the same slice number or the same virtual carrier.

To achieve that, a time slice selector (102) at the input of themodulator (see FIG. 5) collects all frames from the different satnets.The incoming frames are referred to as virtual carrier baseband frames,i.e., baseband frames belonging to a particular virtual carrier. Thetime slice selector multiplexes the frames onto a single bit streamtowards the modulation and encoding means (103) on a first come firstserve basis, provided some time slicing related conditions aresatisfied, which are detailed next. More specifically, at the input sidefrom the time slice selector the received baseband frames are stored instorage means. At the output side of the time slice selector, afteroutputting a selected baseband frame to the encoding and modulationmeans (see Action 1 in FIG. 6), the time slice selector updates (Actions2 and 3 in FIG. 6) a list of allowed time slice numbers and consequently(as virtual carriers are related to disjunct sets of time slice numbers)also a list of allowed virtual carriers (a virtual carrier is allowed ifit is associated to at least one allowed time slice number). Then theoldest virtual carrier baseband frame in the memory that is from anallowed virtual carrier (Action 4-1-a in FIG. 6), is assigned an allowedtime slice number (Action 4-1-b in FIG. 6) and output to the encodingand modulation means (Action 1 in FIG. 6). The baseband frames output bythe time slice selector (the single output bit stream) are referred toas slice numbered baseband frames, i.e., baseband frames with aparticular time slice number. The time slice selector can only put atime slice number on the list of allowed time slice numbers providedthat the previous outputted frame with the same slice number (or fromthe same virtual carrier to avoid reordering, see above) was allowedsufficiently long ago, i.e. there must be a sufficient time intervalbetween two frames of the same slice number (or same virtual carrier).In other words, as the transmission is continuous, there must have beensufficient symbols transmitted in between two frames of the same slicenumber (or same virtual carrier). If there is no single time slicenumber on the allowed list of time slice numbers, then “dummy data” willbe output from the time slice selector, as explained further. In apreferred embodiment the oldest virtual carrier baseband frame in thememory (i.e., which was stored longest ago) that is from an allowedvirtual carrier (Action 4-1-a in FIG. 6) is selected. In anotherembodiment, a virtual carrier can be prioritized such that virtualcarrier baseband frames from that virtual carrier are output first ifavailable.

In the encoding and modulation means, the slice numbered baseband framesare not reordered anymore and no new frames (e.g. dummy frames) areinserted anymore in between the slice number baseband frames. The latteris to avoid more time than needed between two frames of the same sliceor virtual carrier. This limitation is different from state of the artmodulators. E.g. modulators implementing the DVB-S2(X) standard insertdummy PLFRAMES, see FIG. 1 in ETSI EN 302 307 and Sec. 5.5.1 in ETSI EN302 307. The task of the dummy frame insertion there was to make surethat at each symbol time the transmit filter (referred to as BB filterin ETSI EN 302 307) had access to valid I/Q symbols in order to have aclean and stable spectrum and a constant power of the RF signal. Inother words, when the FEC encoder did not yet finish the encoding of aBBF frame to a FEC frame (see ETSI EN 302 307), e.g. because there isnot sufficient data to transmit, the PLFramer inserts a valid dummyframe such that the spectrum remains clean and the output powerconstant. As such, all receivers also remain in lock at the receiverside.

The insertion of dummy PLFRAMEs can be prevented in the proposedimplementation because the time slice selector is part of the modulatoritself, in the same clock domain, where it can receive a getFrame( )command (i.e., a request) from the encoding and modulation means. ThegetFrame( ) command is the result of backpressure that starts in thebaseband (BB) filter and goes back until the time slice selector. Itmakes sure not too much data is sent at once (and so avoids bufferoverflow) but also that data is sent when needed such that the FECencoder always has an available frame and no dummy frame insertion isneeded. If no data baseband frames are available, the time sliceselector itself can insert dummy frames or data frames with dummy data,as such taking over the dummy insertion task of the PLFramer.

In another embodiment of the invention the encoding and modulation meansdo not send a request to the time slice selector. For example, in casesufficient buffering is present in the modulator, the time sliceselector can output the baseband frames at a rate fast enough to avoidtoo much queuing in its storage means. As the sum of the virtual carriersymbol rates is typically slightly smaller than the physical carriersymbol rate, this rate should not overflow buffers in the encoding andmodulation means. It can of course happen that a frame is not ready whenthe pulse-shaper needs it, such that the PLFramer inserts dummy framesfrom time to time. Such dummy frames do not cause a throughput violationin any receiver, as only the time duration between two data basebandframes is increased. It may only create a small additional throughputdrop as such dummy frame may be the consequence of jitter in themodulator rather than being needed for the time sliced transmission.

The slice numbered baseband frames are modulated onto a RF waveform. Thetime slice selector sequentially selects baseband frames and time slicenumbers for those frames which can then be modulated on the widephysical carrier. The oldest baseband frame (BBF) in the storage meansof the time slice selector can be determined in multiple ways. One wayis to use its baseband frame (BBF) sequence number. The basebandsequence number is attached to a frame at the moment the frame arrivesin the modulator, before it is stored in the storage means (e.g. a RAM).The number just assigns a number to the frames in the order they havearrived at the modulator. It allows determining who arrived first. Tohandle the sequence number wrapping, the most significant bit (MSB) ofthe sequence number of the last transmitted BBF to the modulator core isstored. Before sequence number wrapping, the MSB is 1. As long as thereare BBF sequence numbers with the same MSB value, these are older thansequence numbers starting from 0 again. The sequence numbers range from0 to 32767, which is twice the total number of frames that can bestacked after each other in the memory. This avoids wrong detection ofthe oldest BBF in case of overflow conditions. The oldest virtualcarrier baseband frame in the list of allowed virtual carriers is thevirtual carrier baseband frame that has the lowest sequence number withthe same MSB value as the last transmitted BBF to the modulator core.

The selected slice numbered baseband frames are modulated as follows.Each slice numbered baseband frame is encoded by the modulator to aforward error corrected (FEC) frame (called coding), mapped to aphysical layer frame of baseband symbols (called modulation), includinga physical layer header (containing amongst others a start-of-framesequence and an encoded time slice number) and data symbols, and finallypulse shaped to a baseband waveform. Hence, each baseband frame isassociated to a modulation and coding (modcod). In DVB-S2 the number ofcoded bits of “normal” FEC frames equals 64800 bits. Thus, the number ofbits in a baseband frame, before encoding, has a length that depends onthe encoding rate, e.g. 2/3. Also, the number of data symbols in anencoded mapped baseband frame depends on the number of bits that aremapped to a single baseband symbol (e.g., 2 bits for QPSK, 3 bits for8-PSK, 4 bits for 16-APSK constellations, etc). The baseband waveform isfinally upconverted onto a carrier frequency by an IQ modulator. Beforeperforming the pulse shaping, the gateway transmitter typicallyregisters the actual value of a reference clock in the modulator at thetime instant the Start Of Frame (SOF) symbol gets pulse-shaped (see forexample ETSI EN 302 307: “Digital Video Broadcasting (DVB); Secondgeneration framing structure, channel coding and modulation systems forBroadcasting, Interactive Services, News Gathering and other broadbandsatellite applications”, annex G.5). This value is then inserted in aplaceholder for the network clock reference (NCR) of a subsequent framebefore encoding (see for example FIG. 7 in Sec. 6.1 of ETSI EN 301 790V1.5.1 (2009-05), “Digital Video Broadcasting (DVB); Interaction channelfor satellite distribution systems”), such that all terminals can slaveto this common value, which allows them to synchronize their return linktransmissions. As mentioned before, the present invention is not limitedto ETSI EN 302 307 or DVB-S2(X). Any framed communication system mayapply the approach disclosed in this document.

The virtual carrier baseband frames are buffered in storage means in themodulator, for example on a field programmable gate array (FPGA) or anapplication-specific integrated circuit (ASIC or chip). One way is tohave storage means per virtual carrier. Another way is to keep track ofthe pointers to the baseband frames of a virtual carrier while storingall baseband frames in a global memory. The person skilled in the artcan understand that there are other ways to store and again retrieve theframes of multiple virtual carriers.

The time slice selector can run on a processor or in the FPGA. To updatethe list of allowed time slice numbers (Action 3 in FIG. 6), the timeslice selector maintains (Action 2 in FIG. 6) a counter per slice number(or in addition a per virtual carrier counter in case more than oneslice number is used per virtual carrier and in the presence of alimitation on the time duration between two frames of a different slicenumber but the same virtual carrier) indicating the amount of symbolstransmitted as of and including the last frame of that slice number (orvirtual carrier) was released in the single bit stream. Hence, when aslice number from the list of allowable slices is chosen for a selectedvirtual carrier baseband frame, the counter for that slice number isreset to the corresponding length of the selected virtual carrierbaseband frame (or, equivalently, reset to zero, after which allcounters are increased with the corresponding length of the selectedvirtual carrier baseband frame). Once the counter exceeds a per slicenumber programmable limit, the slice number enters the list of allowedslice numbers. The list of allowed virtual carriers is updated with allvirtual carriers having at least one allowed time slice number. Theoldest virtual carrier baseband frame from a virtual carrier in thatlist is then the next frame to be released on the single bit stream(Action 4-1-a in FIG. 6). An allowed time slice number associated to thevirtual carrier baseband frame is assigned to the selected frame (Action4-1-b in FIG. 6). In case no frame is present in the list of allowedslice numbers, a non-limited frame is transmitted.

The per slice programmable limit used as a basis for allowance to thelist of allowed slice numbers is an indication of the throughput limitspresent in at least one receiver that must decode frames with thoseslice numbers. Such throughput limits are presented and discussedfurther in this description. Those programmable limits, an indication ofsaid throughput limits, can be conveyed to the time slice selector bythe central unit or by any entity aware of the throughput limit of theslice numbers. In another embodiment this programmable limit isconstant, e.g. in case it is known that all receivers at least achieve aminimum particular throughput.

The transmission of the non-limited frame may be implemented in multipleways. E.g., the time slice selector may output a baseband frame thatcorresponds to a non-limited frame to the encoding and modulation means.In another implementation, the time slice selector does not outputbaseband frames corresponding to non-limited frames itself, but it mayoutput one or more state bit to the encoding and modulation meansindicating the number and type of non-limited frames that must beinserted by the encoding and modulation means.

In another embodiment the list of allowed virtual carriers not onlycomprises all virtual carriers having at least one allowed time slicenumber, but for example always comprises a virtual carrier that isdecoded by receivers that are arranged for demodulating and decoding atthe physical carrier symbol rate.

In one embodiment (see FIG. 7) non-limited frames may be frames that canbe demodulated and decoded by decoders which are able to demodulate anddecode consecutive frames at the physical carrier symbol rate. Assume,for example, a physical carrier subdivided in three virtual carriers.The first and the second virtual carrier are for satnets with receiversthat can only deal with a throughput lower than the physical carriersymbol rate. The third virtual carrier, with its dedicatedshaper-encapsulator, can be decoded by a receiver capable of operatingat the physical carrier symbol rate (an “expensive receiver”, so tosay). If no allowable slice number(s) for the receivers for the firstand second virtual carrier is/are found or if no baseband frames forthose receivers are available, available baseband frames for thereceivers for the third virtual carrier can be transmitted (Action 4-2-ain FIG. 7). Then the oldest stored baseband frame from the third virtualcarrier is selected or, in another embodiment, priorities can be takeninto account instead of simply selecting the oldest one, as explainedbefore. Those third virtual carrier baseband frames still get assigned atime slice number (Action 4-1-b in FIG. 7) as it is not allowed to mixtransmissions with and without time slicing. This is because allreceivers need to maintain lock on the continuous carrier, such that theformat of the header (which is different with and without time slicing)must kept constant. In a preferred embodiment an implementation (seeFIG. 6) is simply to set the programmable limit for the time slicenumbers decoded by the receivers that can handle the physical carriersymbol rate to zero or another sufficiently low value, so that thosetime slice numbers are always on the candidate list of time slicenumbers and thus always allowed for transmission. When no data basebandframes are available or allowed to be transmitted, the time sliceselector selects a force-empty frame. A force-empty frame is a slicethat will be rejected by all receivers and that is forced in the carrierby the modulator, such that the modulator is sure about the time betweenslices. For example, there are 256 possible slice numbers, from 0 to255. The force-empty frames can then e.g. be associated to slice 255(Action 4-2 in FIG. 6), while the communication system only allowsterminals to be associated with slice numbers 0 to 254. Obviously, anyother slice number can be chosen for the force-empty frames as long as aterminal cannot be associated with that slice number.

The flow of the time slice selector in the modulator is clarified inFIG. 6 and can be summarized as follows. On the one hand, it receives atits input virtual carrier baseband frames which are assigned a BBFsequence number and consequently stored in the storage means. On theother hand, the time slice selector receives a getFrame( ) command fromthe encoding and modulation means (see FIG. 6 above Action 1). Upon thatcommand, it outputs the selected baseband frame with slice number to themodulation and encoding means (Action 1 in FIG. 6). Then it requests agetSliceAndCarrier( ) to update (Action 3 in FIG. 6) the list of allowedtime slice numbers and thus virtual carriers and determines the oldestallowed virtual carrier baseband frame (Action 4-1-a in FIG. 6) toobtain a time slice number (Action 4-1-b in FIG. 6) and a virtualcarrier baseband frame to be outputted to the encoding and modulationmeans next (Action 1 in FIG. 6). At start-up, the default time slicenumber is that of the force-empty frame, e.g. slice number 255. Uponreception of a getSliceAndCarrier( ), the slice selector knows when saidnext frame will be sent (because it knows the time duration of theprevious outputted frame from the slice selector) and it chooses theoldest frame in storage means for the allowed virtual carriers. If novirtual carrier baseband frame is available in the storage means, aforce-empty baseband frame is sent (Action 4-2 in FIG. 6). The symbollength of the chosen frame, which depends on the modcod of that frame,is looked up (Action 2 in FIG. 6). The counters for each of the timeslice numbers are incremented with that symbol length and the list ofcandidate time slice numbers and consequently the list of candidatevirtual carriers are updated.

Force-empty frames of course occupy symbols in the aggregate physicalcarrier. However, those symbols are not assigned to a virtual carrier asthose frames do not result from a satnet processor or are not directedto any receiver. As a result, the sum of the virtual carrier symbolrates must be lower than the symbol rate of the physical carrier. Hence,there is an inherent inefficiency related to time slicing, which can bevery small however. For example, the aggregate physical carrier size(physical symbol rate) should exceed 100.2% of the sum of the virtualsymbol rates present in that carrier. In case of three virtual carriers(VCs) of 140 Mbaud, the aggregate symbol rate must be greater than orequal to 420.84 Mbaud.

The type of force-empty frames to be sent is very important. Force-emptyframes must be valid frames, which allow the receivers to maintain lock,even if hundreds of force-empty frames are sent consecutively. Hence, inthe case of DVB-S2X, a force-empty frame should use an officialmodulation and coding (modcod), e.g. for ETSI EN 302 307, with normal orshort FEC codes and with pilots on or off mode. Since receivers aresometimes made by other vendors than transmitters, standardizedtransmission is often used, e.g. DVB-S2X. Hence, the modcod of theforce-empty frames should be one of the official modcods as published inDVB-S2X. Still, many modcods can be chosen and it is important to pickthe right modcod for the force-empty frames in order to have the bestperformance. Hardware measurements yield an optimal data frame errorrate performance if standardized frames with a header and pilot symbolson are used as force-empty frames and when the modulation order is nottoo high. As a consequence, the extreme example of 256APSK frameswithout pilot symbols is not an ideal type of force-empty frames. Whensuch frames are sent, certain receivers may have difficulties tomaintain the synchronization in an optimal way. In general, force-emptyframes that use 256APSK, 128APSK or 64APSK modcods are to be avoided formany receivers. Also the use of a “pilots off” mode (see Sec. 5.5.3 inETSI EN 301 790 V1.5.1 (2009-05)) in force empty frames is betteravoided. Similarly, it is to be avoided to use short frames asforce-empty frames. Adopting these recommendations leads to the bestsynchronization and more correct E_(s)/N₀ monitoring in receivers thatcan only process a lower throughput than the physical symbol rate.

In order to fix the type of force-empty frames, one must firstunderstand its impact on the satellite network (satnet) sizes and morespecifically, on the virtual carrier symbol rate limits. Suppose themaximum coded throughput at the receiver side, per slice, is 258 Mcbps(Mega coded bits per second). At first, it may appear that the satnetsize would then be such that the coded throughput in the satnet is atmost 258 Mcbps assuming that a virtual carrier only is associated to oneslice number. However, due to tiling loss (illustrated below), the codedthroughput limit per satnet is in practice lower. As mentioned before,force-empty frames of course occupy symbols in the aggregate physicalcarrier which already caused the sum of the virtual carrier symbol ratesto be strictly smaller than the aggregate physical carrier symbol rate.In addition, force-empty frames cause tiling loss, as now explained bymeans of an example. Suppose the maximum coded throughput at thereceiver side, per slice, is 258 Mcbps, which will serve as runningexample in this description. This limit is denoted Lim1=258 Mcpbs.Clearly, this throughput limit Lim1 at the receiver side depends on theactual receiver, type of chip, supplier etc. Any other receiver sidelimitation can simply lead to a new limit Lim1. A numerical example isprovided below for DVB-S2X, but the invention is obviously not limitedthereto. Given that a coded frame consists of 64800 bits, at least251.16 μs must pass in this running example before sending a new frameof 64800 bits, as 64800/258 μs=251.16 μs. Hence, in the running examplethe minimum guard time between two consecutive coded frames of the sameslice is 251.16 μs. The worst case tiling loss is that a force-emptyframe is sent just before the guard time has finished. In that case theactual guard time between two consecutive frames is thus 251.16 usincreased with the duration of a force-empty frame transmission. Thetiling loss can be captured by a force-empty efficiency variable, whichequals the ratio of the minimum guard time to the actual worst-caseguard time (being the sum of guard time and the tiling duration). In thecase of two slices per virtual carrier, the guard time of both slices isincreased with the tiling duration. Consider, merely for the sake of theexample, the following two force-empty modcods in the case of DVB-S2X:

-   -   32APSK 5/6 short frames pilots on, which occupy 3492 symbols,        or, at x Mbaud, 3492/x μs, e.g., 11.64 μs at 300 Mbaud.    -   QPSK 1/4 normal frames pilots on, which occupy 33372 symbol, or,        at x Mbaud, 33372/x μs, e.g. 111.24 μs at 300 Mbaud.

As already mentioned, short frames are not ideal as force empty framesto maintain synchronization and for E_(s)/N₀ monitoring, but it is stilluseful to look at this example as it provides insight on the impact oftiling on virtual carrier symbol rate limits and throughput loss.Clearly, the tiling duration of the QPSK normal frame is 10 times longerthan that of the 32APSK short frame. Assuming the 32APSK 5/6 shortforce-empty frame, the worst case force-empty efficiency is (with xdenoting the baud rate)

FE-Eff_lower=251.16/(251.16+3492/x)

which is a lower bound (as it assumes a force-empty frame is sent justbefore the guard time is finished, which is almost never the case) tothe actually observed force-empty efficiency. For any Lim1 throughputlimit, any tiling duration T in number of symbols and any FEC codedlength size F (here 64800), this becomes

FE-Eff_lower=F/Lim1/(F/Lim1+T/x)

Even more in general, in case other throughput limitations exist at thereceiver side, i.e. not only Lim1, but also Lim2, Lim3, etc., anddenoting Lim=min(Lim1, Lim2, Lim3, . . . ), this becomes

FE-Eff_lower=F/Lim/(F/Lim+T/x).

This lower bound for 32APSK short as well as for QPSK normal force-emptyframes, is shown in FIG. 8 for the running example.

To grasp this better, consider the following example, with one virtualcarrier, with 1 or 2 slices:

-   -   carrier size=300 Mbaud    -   the virtual carrier is shaped to a virtual carrier symbol rate        of v Mbaud    -   the virtual data carrier transmits a fixed modcod for the        payload: 32APSK 5/6 pilots on, normal frames.        Summarizing, data frames in the example are 32APSK normal frames        and force-empty frames are 32APSK short or QPSK normal frames.        The question is what the maximum virtual carrier symbol rate v        is so that all slices can be transmitted at a throughput lower        than the receiver throughput limitation Lim, e.g. 258 Mcbps.

The skilled person can retrieve the numbers provided in the followingexample by applying ETSI EN 302 307: “Digital Video Broadcasting (DVB);Second generation framing structure, channel coding and modulationsystems for Broadcasting, Interactive Services, News Gathering and otherbroadband satellite applications”, annex M) and also in DVB-S2X (DigitalVideo Broadcasting (DVB), DVB Document A83-2, Second generation framingstructure, channel coding and modulation systems for Broadcasting.Interactive Services. News Gathering and other Broadband satelliteapplication, Part II: S2-Extensions (DVB-S2X), March 2014). The numberof symbols in a 32APSK normal frame with pilots on equals 13428 symbols.Two normal data frames (in the case of two slices per virtual carrier)of each 13428 symbols (including header and pilots) at 300 Mbaud aretransmitted in 89.52 μs. As a consequence, after transmission of twodata frames, forced empty frames must be transmitted until at least251.16 μs have passed. Thus, fourteen force-empty 32APSK short or twoQPSK normal frames have to be sent, which results in a tiling loss of1.32 μs or 60.48 μs, respectively. In this case the force-emptyefficiency is 251.16/(251.16+1.32)=99.48% and251.16/(251.16+60.48)=80.5%, for 32APSK 5/6 short force-empty frames andQPSK 1/4 normal force-empty frames, respectively.

Without tiling loss the achievable coded throughput would be equal toLim=258 Mcbps for the considered example. However, due to the tilingloss this decreases by the force-empty efficiency. From the reducedcoded throughput the maximum achievable shaped virtual carrier symbolrate can be computed. In this case (transmitting 5 coded bits per symbolfor 32APSK data frames)

-   -   v=258*99.48%/5*2=51.33*2 Mbaud when transmitting 32APSK 5/6        force-empty frames    -   v=258*80.5%/5*2=41.5*2 Mbaud when transmitting QPSK 1/4        force-empty frames        Clearly, it is of interest to have the highest possible virtual        carrier symbol rates as it determines how many terminals can be        grouped in one satnet and thus how many satnets are needed. The        number of satnets directly translates to capital expenditures,        as it requires satnet processors, multi-carrier demodulators for        the return link (typically one per satnet), etc.

In order to propose clear virtual carrier symbol rate limitations, twomodes of operation have to be distinguished: the physical carrier symbolrate limited mode and the slice limited mode. In the physical carriersymbol rate limited mode the physical carrier symbol rate issufficiently low, such that sending data frames consecutively does notlead to a throughput violation at the receiver side; hence, noforce-empty frames must be sent if sufficient traffic is present (if notsufficient traffic is present, force-empty frames are sent, but as thereis less traffic, this does not lead to buffer overflows). Clearly, allvirtual carrier symbol rate limits can then simply be derived directlyfrom the throughput limitation at the receiver side, e.g. 258 Mcbps. Theforce-empty efficiency is then 100%, resulting in a maximum physicalcarrier symbol rate of v=258*100%/5=51.33 Mbaud per slice. The maximumcarrier baud rate for which the baud rate limited mode is active for32APSK data frames, is

BRmax=258/5*Nr of Total slices per virtual carrier,

or, for two slices per virtual carrier, BRmax equals 103.2 Mbaud.

In general, for data frames with a constellation having l coded bits persymbol and provided that the maximum coded throughput of the receiver isLim, the maximum physical carrier symbol rate in case there is only onevirtual carrier that fully occupies the carrier, is

BRmax=Lim/l*Nr of Total slices per virtual carrier

e.g. 102 Mbaud for 32APSK data frames, Lim=254 Mcbps.Assuming Lim=254 Mcbps and extending the table to other codes andmodulations, one gets

-   -   a) 127 Mbaud when the highest modcod is a 16APSK (l=4) modcod    -   b) 102 Mbaud when the highest modcod is a 32APSK (l=5) modcod    -   c) 85 Mbaud when the highest modcod is a 64APSK (l=6) modcod        in case only one virtual carrier is sent in the aggregate        physical carrier and that virtual carrier contains two slices.        The highest modcod refers to the most efficient coding and        modulation. Clearly, in case some margin is taken, the maximum        symbol rates simply change accordingly. In another embodiment of        the invention, the virtual carrier symbol rate limits are        constant, regardless of the most efficient coding and modulation        used in the particular satellite network. In that case one can        for example limit all satnets to 85 Mbaud, even if the highest        modcod is 16APSK. This simplifies the configuration and        modulator implementation, but however leads to smaller satnets        which provides less opportunity for satellite multiplexing gains        (as explained further in this description).

Sometimes, there is an additional limitation in star networks whereinalso a clock reference and other useful data is sent. More specifically,demodulators sometimes have a queue for each time slice number. Thequeue contains the baseband frames belonging to that time slice number,but for multiple reasons (e.g. maintenance of lock, NCR clock recovery,monitoring, . . . ) the queue can also contain for baseband frames notbelonging to the time slice number, information that also fills thequeue. Since QPSK has the longest frame length, two QPSK baseband framesplus the other information for the frames in between can overflow thosequeues. By limiting the symbol rate this can be avoided. Such a symbolrate limit can for example be 144 Mbaud when the highest modcod is aQPSK (l=2) or 8PSK (l=3) modcod.

Above, the physical carrier symbol rate limits have been proposed toremain within the physical carrier symbol rate limited zone. Obviously,in certain cases the physical carrier rate can be outside that zone,e.g. in the case of transmitting 32APSK data frames with a physicalcarrier symbol rate greater than 102 Mbaud. In that case the virtualcarrier rate has to be strictly smaller than 102 Mbaud as shown below.In order to fill a carrier with a greater physical carrier symbol rate,e.g. 450 Mbaud, more than one virtual carrier can be multiplexed in thesame physical carrier. It is now explained what happens in the slicelimited mode. In case there is more than one virtual carrier perphysical carrier, the physical carrier symbol rate can be assumed higherthan the limit for the physical carrier rate limited zone. Indeed, it isof interest for statistical multiplexing to have satnets as large aspossible. The only reason for making virtual carriers smaller than thephysical carrier through time slicing is because of throughputlimitations at the receiver end. Hence, the slice limited mode isassumed to be active. The force-empty efficiency is thus lower than 100%due to tiling loss, as illustrated above.

Consider again the example of the decoder throughput limitation ofLim=258 Mcbps, i.e., the achievable symbol rate of the virtual carrieris lower than Lim/l*Nr of total slices per virtual carrier, or lowerthan 103.2 Mbaud for 32APSK frames, more specifically 103.2*force-Emptyefficiency. The force-empty efficiency is worse just above the physicalcarrier symbol rate limit, because the time duration of a forced emptyframe is larger for lower baud rates. For example, at 103.21 Mbaud, theworst-case force-empty efficiency is 88.13% (43.72%) when sending 32APSK5/6 (QPSK 1/4) force-empty frames.

The lowest force-empty efficiency can be found by considering the fullforce-empty duration as tiling loss at the lowest possible physicalcarrier symbol rate where force-empty frames have to be sent to preventthroughput violations. If only one slice is adopted, the physicalcarrier symbol rate is lower than 51.33 Mbaud, the 258 Mcbps limitationis never violated and force-empty frames are never sent. Hence, thelongest possible force-empty duration is just above 51.33 Mbaud; e.g.for 51.34 Mbaud, in the absence of other virtual carriers (i.e., thereis only one virtual carrier or the other virtual carriers have notraffic), the force-empty efficiency is 78.69% (27.87%) when sending32APSK 5/6 (QPSK 1/4) force-empty frames.

Summarizing, given the impact of “long” forced empty frames on theforce-empty efficiency and the inferior lock behaviour of high orderconstellations, it is clearly preferable to choose short type of framesfor the force-empty frames.

From the above, the shortest force-empty frame with a low modulation andcoding is found most appropriate. Therefore, it is logical to use normaldummy frames (see ETSI EN 302 307-1: “Digital Video Broadcasting (DVB);Second generation framing structure, channel coding and modulationsystems for Broadcasting, Interactive Services, News Gathering and otherbroadband satellite applications; Part I (DVB-S2)”, Sec 5.5.1) asforce-empty frames in the case of using DVB-S2X. Dummy frames are frameswhere the payload symbols all consist of I=1/sqrt(2) and Q=1/sqrt(2). InDVB those are of course still scrambled by the DVB-S2(X) physical layerscrambling.

Some implementations from the demodulator include signaling per receivedframe to the output of the demodulator, for NCR recovery and forstatistics monitoring. This may yield problems at high symbol rates andwhen using short frames, as a lot of signaling per second is output fromthe demodulator. In a particular demodulator implementation, overflow onthe signaling bus can occur as of 380 Mbaud, when using dummy frames.Hence, in the case of using DVB-S2X, another force-empty frame thandummy frames is needed above 380 Mbaud. Short frames in DVB-S2X are notideal for synchronization and E_(s)/N₀ monitoring. To have short DVB-S2X“normal frames”, a higher order modulation and coding is employed.However, 64APSK and higher also leads to synchronization problems andE_(s)/N₀ monitoring. The modulation and coding 32APSK is borderline andonly 20% shorter than 16APSK normal frames. As a result, most preferredis to take 16APSK normal force-empty frames. Note that the inventionalso allows 32APSK or 8PSK normal frames to be used.

In summary, it is proposed to adopt

-   -   dummy force-empty frames below a particular physical carrier        symbol rate    -   16-APSK force empty normal frames above that particular physical        carrier symbol rate        That particular physical carrier symbol rate is around 380 Mbaud        for a particular receiver, but any other receiver may require        another particular symbol rate.

Below 380 Mbaud (force empty duration is 3420 symbols), the worst-caseforce-empty efficiency (just above the physical carrier symbol ratelimits provided above) is 86.4%, 88%, 90% for 64APSK (for physicalcarrier symbol rate 85.1 Mbaud), 32APSK (for physical carrier symbolrate 102.1 Mbaud) and 16APSK (for physical carrier symbol rate 127Mbaud), respectively. Above 380 Mbaud (16APSK force-empty frame durationis 16776 symbols) the worst-case force-empty efficiency (at the physicalcarrier symbol rate of 380 Mbaud) for 32APSK data frames is 85.25%. Theworst-case force-empty efficiency of other modcods is not lower forother type of data frames than those above for dummy frames.

Above the worst-case, force-empty efficiencies have been derived in theslice limited zone without considering the impact that virtual carriersmay have on each other. However, virtual carriers also have an impact oneach other, which further limits the satnet sizes. The following exampleis considered to make this clear. Two virtual carriers (VCs), with oneslice each, with 32APSK data frames can be transmitted each with virtualcarrier symbol rate of 51 Mbaud in a physical carrier of 102 Mbaud. Noforce-empty frames are sent.

However, this is not the case when one of the virtual carriers has32APSK data frames and the other virtual carrier has QPSK data frames(e.g. because of a cloud in between the satellite and the receivers ofthat other virtual carrier, limiting the signal-to-noise ratio and thusthe achievable modulation and coding). More specifically, assume VC1 hasQPSK frames and VC2 32APSK frames. From the above, the maximum virtualcarrier symbol rate is known to be 258/5=51.6 Mbaud for 32APSK and258/2=129 Mbaud for QPSK. Suppose the overall physical carrier symbolrate is 100 Mbaud and both VC1 and VC2 are shaped to 50 Mbaud. Supposethe 32APSK frames arrived first through the Ethernet interface at themodulator (thus are the “oldest” frames) and then the QPSK frames arriveover the same Ethernet interface. Suppose that the shaper-encapsulatorbursts every 10 ms all frames to be sent in those 10 ms to themodulator, at a very high speed, after which the time slice selectorstores those frames and releases them to the modulation and coding meansat the speed of the achieved virtual carrier symbol rate. In this case,for each virtual carrier, the modulator gets bits from the respectiveshaper-encapsulators corresponding to 500000 symbols (such that 50e6symbols are transmitted every second). The slice selector alternatesbetween the QPSK and 32APSK frames because it cannot send twoconsecutive 32APSK frames. Since a QPSK frame is 2.48 times longer thana 32APSK frame, this means the QPSK frame gets, during a time period oft1 seconds, an average symbol or baud rate of ABRVC1([0−t1])=71.3 Mbaudand the 32APSK frames gets ABRVC2([0−t1])=28.7 Mbaud. Consequently,after t1=10/(71.3/50)=7.01 ms, the QPSK frames are all sent (as theshaper-encapsulator of VC1 was shaped to 50 Mbaud, yielding 500000symbols, which are all output from the slice selector after 7.01 ms),but only 28.7/50*7.1/10=40.75% of all 32APSK frames have been sent.After t1 seconds the slice selector must send forceEmpty frames inbetween the 32APSK data frames in order to guarantee that the codedthroughput of the 32APSK frames does not exceed 258 Mcbps. Worst case,the force-empty efficiency at 100 Mbaud is 88% with dummy force-emptyframes, yielding an achievable virtual carrier symbol rate of 45.45Mbaud. Hence, after 10 ms, only 40.75+49/50*2.9/10=69.17%, of the 32APSKframes have been sent. Consequently, a shaped baud rate of 50 Mbaud forboth virtual carriers is not achievable, while it was achievable for twoVCs with 32APSK data frames.

This observation gets less bad with more than two virtual carriers, asillustrated next. Consider for example three virtual carriers of each 50Mbaud in a 150 Mbaud carrier with two possible combinations of dataframes for VC1, VC2 and VC3, respectively: QPSK1, QPSK2, 32APSK, orQPSK, 32APSK1, 32APSK2. In the first configuration the slice selectorfirst alternates 32APSK and QPSK1 frames until the QPSK frames aretransmitted (at 71.3% of 150 Mbaud or 106.96 Mbaud), i.e., after10/(106.96/50)=4.67 ms. Then, the slice selector alternates 32APSK andQPSK2 frames during another 4.67 ms. Now, 9.35 ms have passed. Untilthen, the 32-APSK frames have been sent at 150−106.96=43.04 Mbaud, suchthat 43.04/50*9.35/10=80.47% of the frames have been sent, which isalready more than the 69.17% of frames sent with two virtual carriers.In the last 0.65 ms 53.18 Mbaud can be transmitted of 32-APSK frames, sothat in total 80.47+53.18/50*0.65/10=87.4% of the 32APSK frames havebeen sent. In the second configuration, QPSK, 32APSK-1, 32APSK2, theslice selector alternates 32APSK1, 32APSK2, QPSK frames; hence, QPSKframes get 2.48/4.48 of 150 Mbaud, or 55.4% of 150 Mbaud=83.1 Mbaud,while 32APSK frames get 150−83.1/2=33.45 Mbaud. The QPSK frames are alltransmitted after 50/83.1*10 ms=6.02 ms. Then, the slice selectoralternates 32APSK1, 32APSK2 frames at a worst case force-emptyefficiency of 91.68% or 47.3 Mbaud, during 3.98 ms. In total, theachieved symbol rate or VC1 and VC2=6.02/10*33.45+3.98/10*47.3=38.95Mbaud or 77.8% of the total frames to be sent, also better than the69.17% of the frames transmitted in the case of two virtual carriers.From to the above numerical examples the person skilled in the artunderstands the worst case is that of two virtual carriers where thethird virtual carrier only consists of two QPSK frames. The mechanism inthe presented example is two-fold:

-   -   the actual instantaneously achieved virtual carrier symbol rate        of virtual carriers is higher for lower modulation orders    -   Due to the unequal instantaneously achieved virtual carrier        symbol rates, the lower modulation orders are more quickly        transmitted, after which the higher order modulation order        suddenly are transmitted at a larger rate (e.g. the full        physical carrier symbol rate when there are only two virtual        carriers) which is typically outside of the physical carrier        symbol rate limited zone.

With two slices per virtual carrier, the situation becomes less severe,as the slice selector will alternate 32APSK slice 1, 32APSK slice 2 andQPSK frames. Furthermore, the physical carrier symbol rate is higher asof which the force-empty efficiency is better, typically above 95%.Repeating the above example for two virtual carriers where the othervirtual carrier has QPSK frames, the person skilled in the art caneasily verify from the above that the force-empty efficiency for 32APSKand 64APSK data frames is roughly 90% and 83%. Hence, the virtualcarrier symbol rate that can be achieved by satnets with 32APSK dataframes or 64APSK data frames can be at most roughly 90% and 83% ofBRmax, respectively.

Taking into account the reduction in virtual carrier symbol rates in thecase of multiple virtual carriers, the following table: is obtained:

-   -   a) 114 Mbaud when the highest modcod is a 16APSK (l=4) modcod    -   b) 90 Mbaud when the highest modcod is a 32APSK (l=5) modcod    -   c) 69 Mbaud when the highest modcod is a 64APSK (l=6) modcod        At physical carrier symbol rates higher than 380 Mbaud, the        force-empty frames become 16APSK force empty frames. However,        many more VCs are used to fill the physical carrier of 380        Mbaud, with less stringent consequences due to unequal        instantaneous symbol rates. Numerical simulations show that only        the virtual carrier symbol rate for 32APSK modcods has to be        reduced above 380 Mbaud, more specifically to 80 Mbaud.

The description above allows implementing an earth station transmitterdevice and satellite communication system comprising such an earthstation transmitter device that forms a modular, simple andcost-efficient extension of prior art solutions so that they become ableof applying time slicing. In addition, the implementation allows for adynamic configuration of the virtual carrier symbol rates by the centralunit. As mentioned before, in a more advanced embodiment the centralunit can dynamically change the virtual carrier symbol rates within aphysical carrier taking into account congestion, jitter and otherimportant constraints in the network. To avoid buffer overflow in themodulator, it is important to first lower the symbol rate of the virtualcarriers that get smaller, after which the symbol rate of the virtualcarriers that get bigger can be increased. This is referred to asdynamic load balancing, or, dynamically varying the load to virtualcarriers in order to avoid congestion.

Dynamic load balancing can be applied seamlessly with the presentedconcept of virtual carriers, because all receivers are locked on theaggregate physical carrier whose symbol rate is not changed. In fact,only the amount of time slices selected by receivers is changed, havingno effect on a lock/unlock behaviour. In the case of transmittingmultiply physical carriers, in order to serve each satnet with adedicate physical carrier, this seamless load balancing would not bepossible as the symbol rates of the physical carriers would be changed,causing receivers to go out of lock.

The dynamic load balancing leads to significant capacity gains which wewill illustrate with a numerical example. In a satnet traffic tomultiple terminals (earth station receiver devices in the FWD link) ismultiplexed in the same virtual carrier. In a broadband network eachterminal typically serves multiple broadband users. For example, such aterminal can be a WiFi tower in a village or an airplane, servingmultiple users logging to the WiFi hotspot. The statistical multiplexinggain presented below exploits the fact that the probability of bufferingdata beyond an acceptable limit drops dramatically as the number ofmultiplexed sources increases (B. Maglaris et al., 1988, Performancemodels of statistical multiplexing in packet video communications, IEEEtransactions on communications, 36(7), pp. 834-844). More specifically,internet traffic is “bursty”—there are periods when a user downloads ata high rate, but often users download almost nothing. Because ofstatistical independence of multiple users it is unlikely that allsources are simultaneously downloading and, thus, designing a server toserve at a rate corresponding to the maximum sum rate of all the sourceswould be very wasteful.

For example, a single user consumes on average 20 kbps with a peakdownload rate of 2 Mbps. That is, the peak rate is 100 times the averagerate. If a single user would be served with a dedicate carrier, and thepeak rate would be subject to a service level agreement (SLA), than theaverage use of this carrier would be only 1%. In practice, the contractwith the user specifies a probability of 95% that its SLA is achieved.Assuming a Poisson distribution of the user traffic (J. Cao et al.,2003, Internet traffic tends toward Poisson and independent as the loadincreases, Nonlinear estimation and classification (pp. 83-109),Springer, New York), it can be derived that the average use of thiscarrier would be only 4.8%. Hence, if the average rate of my network is20 kbps, a carrier of 416.66 kbps needs to be provisioned, which is veryinefficient.

As mentioned above, the probability that two users ask a peak rate atthe same time is small. Due to the 95% specification that an SLA must beguaranteed, the average use of a carrier increases to 6.6% if two userswith each 20 kbps are multiplexed in the same carrier. Morespecifically, a carrier of 606 kbps is needed to carry on average 40kbps, while achieving the peak rate SLA in 95% of the time.

Now consider the difference between six virtual carriers of 80 Mbaudeach (in a physical carrier of 480 or 481 Mbaud) with the alternative ofgenerating six physical carriers of 80 Mbaud each. In the latter eachsatnet conveys 80 Mbaud. Depending on the modcod this corresponds to anamount of Mbps. For example, with an average efficiency of three bitsper symbol, this satnet conveys 240 Mbps. From the statisticalmultiplexing theory, it can be derived that this peak rate allowscarrying 83.4% of average throughput, or, 200 Mbps can be carried onaverage, allowing to multiplex 10.000 users of each 20 kbps, whileachieving their 2 Mbps SLA 95% of the time. Hence, the six physicalcarriers of 80 Mbaud could serve 60.000 users in total.

In the presented invention dynamic load balancing allows a statisticalmultiplexing effect over the entire physical carrier, as a virtualcarrier can resize in the case of congestion, as it is unlikely that allvirtual carriers are congested at the same time. Hence, assuming againan efficiency of three bits per symbol, and thus considering the totalpeak rate of 480*3=1.44 Gbps, leads to an achievable 92.9% of thatcarrier for average throughput, or 1.34 Gbps, allowing to serve 67.000users in total, or 11.66% more than without dynamic load balancing withtime slicing.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, such illustration and descriptionare to be considered illustrative or exemplary and not restrictive. Theforegoing description details certain embodiments of the invention. Itwill be appreciated, however, that no matter how detailed the foregoingappears in text, the invention may be practiced in many ways. Theinvention is not limited to the disclosed embodiments.

Other variations to the disclosed embodiments can be understood andeffected by those skilled in the art in practicing the claimedinvention, from a study of the drawings, the disclosure and the appendedclaims. In the claims, the word “comprising” does not exclude otherelements or steps, and the indefinite article “a” or “an” does notexclude a plurality. A single processor or other unit may fulfil thefunctions of several items recited in the claims. The mere fact thatcertain measures are recited in mutually different dependent claims doesnot indicate that a combination of these measures cannot be used toadvantage. A computer program may be stored/distributed on a suitablemedium, such as an optical storage medium or a solid-state mediumsupplied together with or as part of other hardware, but may also bedistributed in other forms, such as via the Internet or other wired orwireless telecommunication systems. Any reference signs in the claimsshould not be construed as limiting the scope.

1.-21. (canceled)
 22. An earth station transmitter device arranged forgenerating a signal to be transmitted to a plurality of earth stationreceiver devices of a satellite communication system, said earth stationtransmitter device comprising: a plurality of shaping means, eacharranged for shaping for data traffic to a different subset of earthstation receiver devices a symbol rate, so obtaining for each subset agroup of time slices outputting at said symbol rate for said group aplurality of baseband frames for said group, and for encapsulating saiddata traffic, a modulator comprising a time slice selector arranged forreceiving and storing said groups of time slices outputted by saidplurality of shaping means, for selecting a stored baseband frame of agroup of time slices from a list of allowable groups of time slices asnext frame to be multiplexed on a single stream, and for assigning tosaid selected baseband frame from a list of allowable time slicenumbers, a time slice number associated to the group of time slices towhich said selected baseband frame belongs, said modulator furthercomprising encoding and modulation means to convert said single streamconsisting of baseband frames into symbols of a continuous physicalcarrier to be transmitted at a physical carrier symbol rate greater thanor equal to the sum of said symbol rates of said groups of slices, acentral unit arranged for conveying to each shaping means of saidplurality the symbol rate of its group of time slices and for conveyingto said modulator a list of possible time slice numbers for each of saidgroups of time slices.
 23. The earth station transmitter device as inclaim 22, wherein said selected baseband frame is the baseband framefrom said list of allowable groups of time slices that was storedlongest ago.
 24. The earth station transmitter device as in claim 22,wherein said central unit is also arranged for conveying to said timeslice selector indications of throughput limits per time slice number.25. The earth station transmitter device as in claim 22, wherein saidencoding and modulation means is arranged to send a request to said timeslice selector to get said next selected baseband frame.
 26. The earthstation transmitter device as in claim 22, wherein said symbol rate ofat least one of said groups of time slices is upper bounded depending onthe most efficient modulation and coding used in that group of timeslices.
 27. A satellite communication system comprising an earth stationtransmitter device as in claim 22 and a plurality of earth stationreceiver devices, wherein at least one earth station receiver device ofsaid plurality is arranged to demodulate and decode said symbols of saidphysical carrier only at a rate lower than said physical carrier symbolrate, said plurality of earth station receiver devices being divided inat least two subsets, each subset corresponding to a different one ofsaid groups of time slices.
 28. The satellite communication system as inclaim 27, wherein said time slice selector is arranged for selecting anon-limited frame to be put in said single stream in case no basebandframe is found with allowable time slice number.
 29. The satellitecommunication system as in claim 28, wherein said non-limited frame isthe oldest stored baseband frame belonging to a group of time slices tobe received by at least one earth station receiver device of saidplurality arranged for demodulation and decoding at said physicalcarrier symbol rate.
 30. The satellite communication system as in claim28, wherein said central unit is arranged to keep a separate list oftime slice numbers for said at least one earth station receiver deviceof said plurality arranged for demodulation and decoding at saidphysical carrier symbol rate.
 31. The satellite communication system asin claim 30, wherein said non-limited frame is a frame that is forcedinto said single stream by said time slice selector and is rejected bysaid plurality of earth station receiver devices.
 32. The satellitecommunication system as in claim 28, wherein said non-limited frame is aframe that is forced into said single stream by said time slice selectorand is rejected by said plurality of earth station receiver devices,said frame comprising a header identifying said frame as a dummy frame.33. The satellite communication system as in claim 32, wherein saidframe forced into said single stream is a dummy frame if said physicalcarrier symbol rate is below a given threshold and a 16APSK or 32APSKframe with dummy data above said given threshold.
 34. The satellitecommunication system as in claim 33, wherein said frame forced into saidsingle stream is a 16APSK or 32APSK normal frame with dummy data. 35.The satellite communication system as in claim 34, wherein said 16APSKor 32APSK normal frame is a prestored, already coded frame with dummydata.
 36. The satellite communication system as in claim 27, whereinsaid list of allowable time slice numbers comprises time slice numbersfor which a counter exceeds a first threshold linked to a giventhroughput limitation per time slice on an earth station receiver deviceof said plurality, said counter arranged to count a number of symbolstransmitted as of and including the last outputted baseband frameassigned to said time slice number.
 37. The satellite communicationsystem as in claim 36, where said list of allowable groups of timeslices comprises groups of time slices having at least one time slicenumber on said list of allowable time slice numbers.
 38. The satellitecommunication system as in claim 36, where said list of groups of timeslices comprises groups of time slices for which at least one furthercounter exceeds a second threshold related to a jitter specification onan earth station receiver device of said plurality that can lead toframe reordering, said second counter arranged to count a number ofsymbols transmitted as of the last outputted baseband frame assigned tosaid group of time slices.
 39. The satellite communication system as inclaim 27, where said central unit is arranged to indicate to a firstshaping-encapsulation means an increased symbol rate, based oncongestion of a corresponding satnet, and to indicate a decreased symbolrate to at least one other shaping-encapsulation means, such that saidphysical carrier symbol rate is greater than or equal to the sum of thesymbol rates of all groups of time slices.
 40. The satellitecommunication system as in claim 39, where said central unit is arrangedto first indicate a decreased symbol rate to said at least one othershaping-encapsulation means, before indicating an increased symbol rateto said first shaping-encapsulation means.